1. Field of the Invention
This invention relates to a doubly resonant optical parametric oscillator particularly to a doubly resonant optical parametric oscillator whose wavelength is continuously tunable.
2. Description of the Prior Art
An optical parametric oscillator capable of providing a frequency-tunable coherent beam source (laser beam source) can be used as a light source in spectrometry, photochemistry, physical property research, optical data processing, optical control and optical communications, as well as in optical computed tomography as a tool for collecting optical biometric information, and is also expected to find applications in various branches of medicine and medical environments. Use in fields of quantum optics, such as for generation of frequency-tunable non-classical beams, is also possible.
The ordinary optical parametric oscillator conventionally has a structure obtained by inserting a nonlinear material (nonlinear crystal) into an optical resonator composed of two mirrors. When excited by a coherent beam [pump (excitation) beam, angular frequency xcfx89p], it splits the angular frequency xcfx89p of the coherent beam, at an arbitrary frequency ratio dependent on the phase matching condition to produce a signal wave of angular frequency xcfx89s and an idler wave of angular frequency xcfx89i.
Based on optical resonator structure, optical parametric oscillators are divided into the doubly resonant optical parametric oscillator (DRO), which resonate both the signal wave and the idler wave, and the singly resonant optical parametric oscillator (SRO), which resonates one or the other of the light waves. Although the DRO has the advantage of a much lower threshold than the SRO, it has the drawback of unstable oscillating performance and, as discussed further in the following, is incapable of continuous frequency tuning. On the other hand, the SRO requires a high-output laser as a pump beam source because of its high threshold. A particular disadvantage of the SRO is therefore the difficulty of realizing optical parametric oscillation with a continuous wave (CW) beam source.
The relationship among the pump beam angular frequency xcfx89p, the signal wave angular frequency xcfx89s and the idler wave angular frequency xcfx89i expressed by the following Equation 1 has to be established to realize optical parametric oscillation.
xcfx89pxe2x88x92xcfx89s+xcfx89ixe2x80x83xe2x80x83Eq. 1
In a DRO, therefore, an attempt to achieve frequency tuning by varying the resonator optical lengths cannot succeed because simultaneous increase or decrease of both the xcfx89s and xcfx89i resonant frequencies makes it impossible to maintain the relationship of Equation 1. A DRO therefore cannot produce an output capable of continuous frequency tuning because oscillation becomes intermittent during the frequency tuning owing to mode hopping.
In contrast, frequency tuning of an SRO can be achieved without upsetting the relationship of Equation 1 because only one or the other of the signal wave and the idler wave is resonated.
In order to achieve DRO frequency tuning it has therefore been proposed to configure separate optical resonators for the signal wave and the idler wave so that the signal wave and the idler wave can be separately resonated.
In a DRO provided with separate optical resonators for the signal wave and the idler wave, however, continuous resonant frequency tuning is difficult to conduct because the optical lengths of the optical resonators have to be controlled independently.
This invention was accomplished in light of the foregoing circumstances. An object of the invention is to provide a doubly resonant optical parametric oscillator (DRO) that enables simultaneous resonance of the signal wave and the idler wave and continuous wavelength tuning without upsetting the relationship of Equation 1.
Another object of this invention is to provide a DRO that achieves high oscillation stability using either a continuous wave beam source or a pulse beam source.
For achieving this object, this invention provides an optical parametric oscillator comprising:
a nonlinear element for converting an incident pump beam of angular frequency xcfx89p into a combination of a signal wave of angular frequency xcfx89s and an idler wave of angular frequency xcfx89i at an arbitrary angular frequency ratio,
beam splitting means (e.g., a polarizing beam splitter) for splitting the combination exiting the nonlinear element into the signal wave of angular frequency xcfx89s and the idler wave of angular frequency xcfx89i,
signal optical resonance means (e.g., an input/output mirror, nonlinear crystal, polarizing beam splitter, relay mirror, and common end mirror) for resonating the signal wave of angular frequency xcfx89s split by the beam splitting means at a resonator optical length Ls between an input mirror and an end mirror, and
idler optical resonance means (e.g, an input/output mirror, nonlinear crystal, polarizing beam splitter, relay mirror and common end mirror) for resonating the idler wave of angular frequency xcfx89i split by the beam splitting means at a resonator optical length Li between an input mirror and an end mirror,
the resonator optical length Ls of the signal optical resonance means and the resonator optical length Li of the idler optical resonance means being set to satisfy the relationship Li/Ls=xcfx89i/xcfx89s,
a signal wave reflection side functioning as the end mirror of the signal optical resonance means and an idler wave reflection side functioning as the end mirror of the idler optical resonance means being faced in opposite directions to form a common end mirror, and
continuous frequency tuning being conducted while maintaining signal wave and idler wave oscillations by moving the common end mirror within a range wherein a maximum variation range xcex4L(Max) satisfies xcex4L(Max)=2Cxcex3/[xcfx89sxe2x88x92xcfx89pxc3x97Ls/Ls+Li)] (where C is the velocity of light and xcex3 is an optical resonator loss parameter).
As pointed out above, continuous oscillation wavelength tuning can be achieved without upsetting the relationship of Equation 1 by causing the signal wave angular frequency xcfx89s, idler wave angular frequency xcfx89i, resonator optical length Ls, and resonator optical length Li to satisfy the relationship Li/Ls=xcfx89i/xcfx89s and the maximum variation range of common end mirror to satisfy 2Cxcex3/[xcfx89sxe2x88x92xcfx89pxc3x97Ls/(Ls+Li)].
Since the signal optical resonance means and the idler optical resonance means are separately configured and share only the common end mirror, a parametric oscillator exhibiting a high degree of oscillation stability can be provided.
The above and other objects and features of the invention will become apparent from the following description made with reference to the drawings.